The present invention relates to an image forming apparatus which enhances stable transfer of an image to a paper sheet and reliable transport of the paper sheet in the event that transfer means is accelerated relative to photoconductive means to reduce a copying time.
A color electrohotographic copier of the type having optics for scanning, photoconductive means, and transfer means is known in the art. In this type of copier, the optics repetitively scans a color original document to sequentially form latent images associated with the document on the photoconductive means which is rotating. The latent images are each developed by color toner, and the resulting toner images are sequentially transferred to a paper sheet which is loaded on the transfer means which is rotating in contact with the photoconductive means in a predetermined transfer region. The paper sheet undergone such image transfer is separated from the transfer means by separator means which is positioned near the transfer means. Finally, the paper sheet is fed to fixing means to complete a color copy. It has been customary to implement the photoconductive means with a photoconductive drum and the transfer means with a transfer drum. The photoconductive drum is sequentially exposed to a plurality of separated color components by the optics, whereby electrostatic latent images each being associated with a respective one of the color components are provided on the photoconductive drum. The latent images are each developed by toner of a particular color which is complementary to the latent image, and the resulting toner images are sequentially transferred to a paper sheet which is retained on the transfer drum.
A prerequisite with the above-described type of color copier is that the copier be free from the deviation of color components on a paper sheet which is detrimental to the quality of reproduction. This requirement is generally met by designing the circumferential dimension of one of the photoconductive and transfer drums to be an integral multiple of that of the other and interconnecting the two drums by gears or the like which encounter a minimum of backlash to drive them together at a constant speed. Such a drive system, however, causes the transfer drum to simply follow the rotation of the photoconductive drum and therefore rotate at a constant speed with no regard to the size of a paper sheet, resulting in the copying time being constant with no regard to the same. That is, it does not contemplate increasing the rotation speed of the transfer drum relative to that of the photoconductive drum after the trailing edge of a paper sheet of comparatively small size has moved away from the transfer region and thereby reducing the copying time.
One approach heretofore proposed to reduce the copying time is driving the transfer and photoconductive drums by individual drive sources and providing positioning disks on opposite sides of the photoconductive drum such that the disks are free to rotate on a common axis. The positioning disks receive a transfer pressure developed between the two drums so that the transfer drum alone may be accelerated during the transfer of a toner image to a paper sheet of comparatively small size. With this scheme, since the rotation speed of the transfer drum is variable independently of the photoconductive drum, it is theoretically possible to accelerate the transfer drum during the interval between the movement of the trailing edge of a paper sheet away from the transfer region and the arrival of the leading edge of the same sheet at the transfer region to thereby make the copying time as short as possible.
A problem with the above-described scheme is as follows. A prerequisite with color copying is that a paper sheet undergone the last one of a sequence of transfer cycles be guided by a separator pawl, a kind of separator means, after its leading edge has been released from a sheet gripper. Assume that the transfer drum is accelerated simply by sensing that the trailing edge of a paper sheet has moved away from the transfer region, and that the paper sheet is of comparatively small size. Then, since the leading edge of the paper sheet has not reached the separator pawl yet, various troubles such as damage to non-fixed toner images and a jam are apt to occur due to substantial resistance ascribable to separation which occurs when the leading edge of the paper sheet moves away from the separator pawl.